Alpha-sulfo-fatty acid ester salts are valuable surfactants which have strong resistance to hard water and can be easily biodegraded. They can be used in daily chemical industry, ore dressing and the like.
The sulfonation of a fatty acid ester is often carried out in a falling film sulfonator by a reaction of the fatty acid ester and sulphur trioxide. The obtained sulfonic acid is often dark in color, which renders the appearance of the final product unacceptable. Therefore, the sulfonic acid is typically subjected to bleaching to make the product color as light as possible. The neutralized sulfonic acid is a paste product containing 30%-50% of the active compound, which is hard to be transported in a large amount and is not suitable to be prepared into a detergent in powder form. Thus, it is preferably dried into granules or powders before being directly formulated with other powdery materials. In addition, the sulfonation reaction inevitably generates an undesirable disodium (or dipotassium) salt. Thus, it is necessary to perform a reesterification in order to reduce the content of the di-salt as much as possible. In order to produce a satisfying alpha-sulfo-fatty acid ester salt, it is necessary to undergo the above steps.
Much efforts have been made on the sulfonation of fatty acid esters in previous processes. Free oil content and color, however, cannot be ensured at the same time. The product color will become dark if the content of free oil is reduced, while the content of the free oil will increase if the color is whitened. In the bleaching process, bleaching and reesterification are mostly carried out in one step, in which hydrogen peroxide (oxydol) is used as bleaching agent in an alcoholic system at a temperature above 90° C., see the Chinese patents No. CN 94115317.7, and CN 00133161.2. The temperature is much higher than the boiling point of the alcohol, which also facilitates the decomposition of hydrogen peroxide, thus rendering the system being recycled under high pressure. The process is also accompanied by some side reactions, which produce such as ethers and the like. The ethers are very dangerous since they are flammable and explosive, being hard to be disposed. In addition, a large amount of the alcohol has negative influence on the bleaching rate and effect. There are also attempts to bleach the sulfonic acid with hydrogen peroxide in a stainless steel equipment in a batch way. However, the bleaching reaction is difficult to control because of the highly viscous product and the rapid exotherm, which negatively affects the bleaching effect and the reproducibility of the product with good performance.
Due to high susceptibility to temperature and high viscosity of the alpha-sulfo-fatty acid ester salts, it is even difficult to dry them into solid form while the color and the disodium (or dipotassium) salt content are ensured.
Contents of the Invention
The object of the invention is to provide a new process for preparing alpha-sulfo-fatty acid ester salts, which is safe, easy to control and produces a product with satisfying color.
The present invention provides a process for preparing alpha-sulfo-fatty acid ester salt surfactants, which consists of the following steps:
(1) sulfonating a fatty acid ester with sulphur trioxide to prepare a crude sulfonic acid;
(2) bleaching the crude sulfonic acid obtained in step (1) by using hydrogen peroxide as bleaching agent;
(3) subjecting the product of step (2) to a secondary esterification using an alcohol as esterifying agent;
(4) neutralizing the product of step (3) with a base; and
(5) drying the product of step (4) to obtain a powdery product.
The process is carried out by employing one or more of the following means:
Means 1: a continuous bleach step at a low temperature.
Means 2: a secondary esterification step by adding a certain amount of alcohol.
Means 3: a neutralization step by using a base alcoholic solution (including a sodium (or potassium) alkoxide solution or a base in an alcohol), or a liquid base.
Means 4: a drying step in a rotating thin-film evaporator.
Preferably, the fatty acid methyl ester is sulfonated with sulphur trioxide in step (1) at a ratio of 1:1.15 to 1:1.35 (preferably 1:1.25) at a temperature between 75° C. and 85° C. (preferably 80° C.). The obtained sulfonic acid is subjected to a two-step aging, the first lasting for 40 to 60 minutes at 80° C.-90° C., and the secondary lasting for 30 to 60 minutes at 70° C.-80° C.
The obtained sulfonic acid is directly bleached with oxydol at a concentration of 25%-50% (preferably 35%). The amount of the oxydol used is 3%-20%, preferably 5%-10%, and more preferably 5%-8% based on the weight of the sulfonic acid.
Preferably, the reactants in step (2) are fed by means of spray.
Further preferably, the reactants in step (2) are mixed homogeneously in a rotor film-scrapping shear mixer.
Further preferably, the heat released during the bleaching in step (2) is removed by cool air blast or room-temperature air blast.
More preferably, the bleaching process is carried out in a continuous way at a temperature between 50° C. and 150° C., preferably between 75° C. and 90° C., by sequentially employing a spray mixing, a homogeneous mixing in a rotor film-scrapping shear mixer and an intensive air cooling step.
The bleached sulfonic acid is subjected to a secondary esterification (step (3)) using an alcohol as esterifying agent. The alcohol can be a C1-C3 alcohol, preferably methanol. The amount of the alcohol used is 75%-200%, preferably 75%-150%, and more preferably 100%, based on the total weight of the sulfonic acid (after bleaching). The secondary esterification is carried out under reflux for 1-6 hours.
In step (4), the sulfonic acid can be neutralized with an aqueous sodium (or potassium) hydroxide solution, an alcoholic sodium (or potassium) hydroxide solution or sodium (or potassium) alkoxide. The neutralization equipment and operation are similar to the bleaching step. Preferably, the base solution used in step (4) is an aqueous or alcoholic sodium hydroxide solution. The base concentration is 5%-50%, preferably 10%-30%. The neutralization temperature is in the range of 40° C.-80° C., preferably 50° C.-60° C.
Preferably, the reactants in step (4) are fed by means of spray.
Preferably, the reactants in step (4) are mixed homogeneously in a rotor film-scrapping shear mixer.
Preferably, the heat released from the neutralization step (4) is removed by cool air blast or room-temperature air blast.
The paste product obtained after neutralization is dried in a rotary thin-film evaporator under vacuum pressure of 50-200 mmHg at a temperature between 50° C. and 90° C.
The preparation process of the present invention is safe, easy to operate and control, and can produce a product with good reproducibility. The product in improved color has a desirable low content of the disodium (dipotassium) salt with little harmful substance retained.